Embodiments of the present disclosure relate to instruments used to locate underground conductive objects such as pipes, cables, and subsurface field generators. The present disclosure is related in particular to those instruments capable of measuring the underground depth of such objects.
Construction contractors and public utility companies commonly use instruments known as “depth-reading line tracers” and “pipe and cable locators” to locate and determine the depth and orientation of buried electrically conductive utilities. The terms “locator” and “instrument” are used interchangeably herein to refer to such instruments. In typical usage, a separate transmitter unit is employed to inject an AC signal current into the utility, thus “energizing” it. This signal current is typically within the frequency range of several hundred Hz to several hundred KHz and sets up a magnetic field around the utility for the locator to sense above ground. Location, orientation and depth of the buried utility are then signaled by visual and/or audible indicators according to the locator's design.
For design purposes, those skilled in the art typically model the magnetic field set up by the energizing current as emanating circular field lines, concentric to the utility. The field strength is inversely proportional to the radial distance from the utility, in accordance with a theoretical field set up by an infinitely long, straight-line conductor. FIG. 1A shows such an exemplary field model 4, emanating from a utility 1, buried at depth 2 under ground level 7. Using the geometric relationships of this circular field model 4 and similar variations of it, designers have devised a number of depth-reading instruments that operate by deriving depth information from measurements taken of the field at multiple locations.
Of particular relevance to the present disclosure is the category of depth reading instruments that operate according to what is commonly known as the “gradient method”. With reference to FIG. 1A, such instruments typically employ a sensor array 10, comprised of an upper sensor 5 and lower sensor 6 which are vertically spaced apart by a fixed distance 3 to measure the magnetic field at their respective vertical positions above the utility. Signals from these sensors 5, 6 are subsequently fed through cables to circuitry (not shown) for further conditioning, processing and computation to determine a depth measurement for the utility 1, which is then presented to the operator. U.S. Pat. No. 4,387,340 and the publication “abc & xyz of locating buried pipes and cables for the beginner and the specialist” by Radiodetection Corp. (1994), both of which are incorporated herein by reference, describe the fundamental operation of such instruments.
Using the geometric properties of the concentric field model described above, and under the conditions that the sensor array is located directly over the buried utility and the lowest sensor is at ground-level, they derive an equation expressing the depth of the buried utility as the product of the spacing between the upper and lower sensors 5, 6 and the ratio of the upper field sensor voltage to the gradient voltage formed by the difference between the lower and upper field sensor voltages. In practice, the spacing between upper and lower sensors 5, 6 is commonly equal to approximately 1 ft. and additional calibration factors and constants are included to compensate for various measurement irregularities and errors.
Conventional locators use sensitive circuitry which is prone to time and temperature drift making depth measurement accuracy a fundamental design challenge. Such drift errors are particularly critical in the formation of the gradient signal used in the depth computation. This gradient signal is formed apart from the signal sources, using two separate signals, whose respective sensor, resonant antenna network, and processing channel errors all contribute to the total gradient error. The location where this difference signal is formed varies according to design. In some instruments, it is formed in the analog circuitry as described in U.S. Pat. Nos. 5,065,098 and 5,231,355, for example. In other instruments, it is computed by an embedded microprocessor after suitable analog signal conditioning, as described in U.S. Pat. No. 4,672,321, for example. The '098, '355, and '321 patents are incorporated herein by reference.
Two basic approaches are used to minimize the aforementioned gradient errors. The first approach is to pay close attention to time and temperature stability issues, with the goal that over time, such differential mismatches will not exceed a fraction of a percent. Such goal is difficult to achieve, however, under the variety of environmental conditions encountered in actual field use over time. In addition, costs associated with product development, use of premium components, and specialized fabrication techniques become prohibitive at some point.
The second approach to minimizing gradient errors multiplexes each sensor's output signal sequentially in time to a shared processing channel. A goal of the second approach is to cancel and minimize errors common to the shared channel. The locator described in U.S. Pat. No. 4,387,340, for example, is typical of those using such an approach. The '340 patent is incorporated herein by reference. Although somewhat more effective than the first approach, the second approach fails to remedy potential errors and mismatches arising from high-Q resonant networks and the sensors themselves. In addition, such time-multiplexed approaches require more time during the measurement process, causing the instrument to respond in a “sluggish” manner. Furthermore, such techniques make the instrument vulnerable to errors resulting from changing conditions during the measurement process.
The present disclosure provides a method and corresponding apparatus used in a locator to measure the depth of buried, current-carrying utilities and subsurface field generators. The present system and method improves the accuracy of such depth measurements by minimizing errors in the field gradient measurement, which is used in the computation of depth and in accordance with the aforementioned “gradient method”.
In an illustrated embodiment of the present disclosure, a direct magnetic field gradient signal and parallel secondary field signal are used in the computation of the depth of the utility. Also in accordance with an illustrated embodiment of the present disclosure, signals are provided by an apparatus, which includes first and second antennas, both rigidly mounted along a common vertical axis inside the locator's housing. The first antenna is a gradiometer formed by two vertically spaced-apart magnetic field sensors. Such sensors are typically spaced apart in the range of 8-20 inches, and are connected in a subtraction configuration to provide a difference signal representative of the field gradient. The second antenna is a single magnetic field sensor located along the same common vertical axis and is aligned in parallel to the gradient sensors. This sensor provides a signal representative of the magnetic field at a point along the axis for use in the depth equation. The direct gradient and secondary signals from the apparatus are sent to circuitry for suitable filtering, amplification and conversion before being processed by an embedded microprocessor. Computation results from the microprocessor are subsequently provided to the operator by electronic means via a display or speaker, for example.
Additional features of the present system and method will become apparent to those skilled in the art upon consideration of the following detailed description of illustrative embodiments exemplifying the best mode of carrying out the present system and method as presently perceived.